The present invention relates to a waveform equalizer for equalizing the waveform of a signal reproduced from a magnetic recording medium.
With signal recording/reproduction system using a ring-shaped head and a longitudinal magnetic recording medium (i.e., magnetic field is oriented along the longitudinal direction of the medium), the isolated reproduced signal has a Lorentz waveform, as shown in FIG. 1A. The discrimination of this reproduced signal is usually performed by a peak detection technique using differentiation.
On the other hand, when a signal which is ideally recorded along the perpendicular direction is reproduced by a ring head from a perpendicular magnetic recording medium (i.e., magnetic field is oriented along the perpendicular direction or the direction of thickness), the reproduced signal becomes point-symmetrical at about the zero amplitude, as shown in FIG. 1B. Therefore, a zero-cross detection technique may be used for data discrimination.
However, in practice, with signal recording and reproduction system using a ring head and the perpendicular magnetic recording medium, a signal reproduced has an asymmetrical waveform obtained by superposing the waveform (referred to "longitudinal waveform" hereinafter) of FIG. 1A on the waveform (referred to "perpendicular waveform" hereinafter) of FIG. 1B, as shown in FIG. 1C. This asymmetrical composite waveform is subjected to data discrimination in accordance with: (1) the peak detection using differentiation like the longitudinal waveform and (2) the maximum inclined point detection by two-time differentiations. The method (1) has a disadvantage in that the peak shift is large due to waveform interference. The method (2) has a disadvantage in that the degradation of S/N ratio becomes large due to the differentiation.
The longitudinal waveform and the perpendicular waveform have a relation of the Hilbert transform, as described by V. B. Minukhin in "Phase Distortions of Signals in Magnetic Recording Equipment", Telecommunications Radio Engineering, Vols. 29-30, PP. 114-120 (referred to reference (1) hereinafter). Using this relationship, data discrimination of a composite waveform of the longitudinal and perpendicular waveforms may be performed without the disadvantages of methods (1) and (2).
A typical method is described by B. J. Langland in "Phase Equalization for Perpendicular Recording", IEEE Trans. on Magn., MAG-18, PP. 1247-1249 (referred to reference (2) hereinafter), according to which a Hilbert-transform filtered waveform of the composite waveform having the longitudinal and perpendicular waveform components and a constant-time delayed waveform of the composite waveform are combined at a predetermined ratio so as to convert the composite waveform to the purely longitudinal waveform for subsequent data discrimination. According to this method, the data discrimination margin can be increased, and the S/N ratio will not be degraded.
However, in reference (2), the detailed arrangement of the Hilbert-transform filter is not described. Reference (2) only describes that a tapped delay line is used. However, when a Hilbert-transform filter is constituted by a tapped delay line in the same manner as in the arrangement of a conventional transversal filter, the delay line is required to have a long delay time and many taps. Therefore, the delay line becomes large in size and costly. For use in magnetic recording/reproducing apparatus of small size and low cost, such a large and costly delay line will not be practical.